Parking brakes or holding brakes are known, by means of which a clamping force holding the vehicle at a standstill is produced. The parking brake is implemented as an electromechanical brake mechanism with an electric brake motor, the motor shaft of which drives a spindle on which sits a spindle nut that is displaced towards a brake piston by the rotational movement of the spindle. The brake piston carries a brake lining and is applied to a brake disk during the drive movement of the brake motor. One such electromechanical brake mechanism is described in DE 10 2012 205 576 A1, for example.
For determining the clamping force, the motor constant of the electric brake motor must be known, the value of which can be subjected to manufacturing tolerances and moreover ageing-related and temperature-related fluctuations. According to DE 10 2012 205 576 A1, for determining the motor constant the voltage and the motor current are measured on the brake motor during a no-load phase, moreover the motor current is determined during a dynamic current change phase.
For building up the clamping force, first a no-load phase must be overcome, during which the spindle nut is displaced towards the disk brake owing to the drive movement of the brake motor, but there is still no contact between the brake lining and the brake disk.